\tracingonline=2
\tracingcommands=2
\tracingmacros=2
\tracingrestores=2

\show\abc@def
\catcode`@=11
\show\abc@def

\catcode`/=0
/show/relax

%errors
\catcode0=10
\catcode555=11
\showthe\catcode0

\catcode0 = 10
\catcode555 = 11
\showthe\catcode 0

\catcode0=10
\catcode0=555
\showthe\catcode0

\end
